1. Field of the Invention
The present invention relates to a novel fluoride single crystal for detecting radiation, a scintillator, a radiation detector using the single crystal and a method for detecting radiation. More particularly, the invention relates to a fluoride single crystal for detecting radiation and for use in a medical diagnosing apparatus such as an X-ray computed tomography (X-ray CT), a positron emission computed tomography (PET), or a time-of-flight positron emission computed tomography (TOF-PET) and to a radiation detector.
2. Background Art
Conventionally, radiation has been employed in a variety of fields such as medical diagnosis and non-destructive inspection carried out in industry. For example, medical diagnosis apparatuses such as an X-ray CT and a PET have been put into practical use. Such apparatuses employing radiation have a detector for detecting radiation such as a γ-ray or an X-ray, and a known detector employs a scintillator.
A scintillator is a substance which radiates, through stimulation by a γ-ray or an X-ray, visible light or an electromagnetic wave having a wavelength in the vicinity of that of visible light. Thus, the scintillator is required to have high density, a short decay period of luminescence, excellent resistance to radiation, etc.
Conventionally, a bismuth germanate (Bi4Ge3O12; BGO) single crystal has been employed as a scintillator material for use in a PET. Japanese Patent Publication (kokoku) No. 62-8472 discloses that a cerium-doped gadolinium silicate (Ce:Gd2SiO5; Ce:GSO) single crystal has also been developed in order to attain excellent performance and is used in practice.
U.S. Pat. Nos. 4,958,080 and 5,025,151, Japanese Patent Application Laid-Open (kokai) No. 9-118593, and other documents disclose that a cerium-doped lutetium oxyorthosilicate (Ce:Lu2SiO5; Ce:LSO) single crystal has been developed on the basis of extensive studies in order to attain further excellent performance and is used in practice as one of the most promising scintillator materials.
As compared with a scintillator for use in a PET, a scintillator for use ,in a TOF-PET is required to possess a higher time resolution. Therefore, at present, CsF is employed in a TOF-PET, and use of BaF2, generally produced as materials for optical lenses, is underway. However, CsF has a drawback of a highly deliquescent nature, and BaF2 has drawbacks that the barium fluoride emits a short-lifetime luminescence employable in a TOF-PET in a UV region, thereby imposing the requirement of a UV detector, and that a delayed, strong light emitted from the fluoride destroys the detector.
In addition to the aforementioned single crystals, a variety of ceramic materials have been investigated for use as a scintillator. Specifically, Japanese Patent Publication (kokoku) No. 59-45022 discloses polycrystalline materials (ceramic materials) such as BaFCl:Eu, LaOBr:Tb, CsI:Tl, CaWO4, and CdWO4. Japanese Patent Application Laid-Open (kokai) No. 59-27283 discloses rare earth metal oxide polycrystalline materials (ceramic materials) having a cubic crystal structure such as (Gd, Y)2O3:Eu. Japanese Patent Application Laid-Open (kokai) No. 58-204088 discloses rare earth metal oxide sulfide polycrystalline materials (ceramic materials) such as Gd2O2S:Pr.
Since such ceramic scintillator materials are produced by sintering a powder material, a variety of approaches have been taken for improving transparency (light-permeability), sinterablity, etc. For example, Japanese Patent Application Laid-Open (kokai) No. 7-188655 discloses that the amount of impurity (particularly phosphate (PO4)) contained in a ceramic phosphor such as Gd2O2S:Pr is reduced to 100 ppm or less, thereby enhancing light output of the scintillator. Japanese Patent Publication (kokoku) No. 5-16756 discloses a ceramic phosphor having an increased density that is produced by adding, as a sintering aid, a fluoride such as LiF, Li2GeF6, or NaBF4 to a rare earth metal oxide sulfide powder and sintering the powder mixture under hot isostatic pressing (HIP) conditions.
As described above, a variety of substances such as oxide single crystals and ceramics have been investigated for practical use as a scintillator. However, no substances having characteristics more excellent than those of Ce:LSO have yet been found, and research in this field is now at a deadlock. Regarding a scintillator for use in a TOF-PET, no substance superior to CsF has ever been proposed.
Notably, since a fluoride single crystal has high light transmittance in a wide wavelength range, a small crystal field, and a negative temperature coefficient of refractive index, the single crystal has been a remarkably promising candidate as a material employable in a laser. However, most of the single crystals other than single-crystals of CsF and BaF2 have not been studied for employment as radiation-detecting materials such as a scintillator. From another aspect, a fluoride single crystal encounters difficulties in production, such as controlling of the atmosphere for production, the temperature at production, the purity of raw materials, and the composition of raw materials. Thus, a large-scale bulk crystal of the fluoride is difficult to produce. Under such circumstances, the present inventors previously developed a technique of producing high-quality single crystals, and produced some fluoride single-crystals. Specifically, Japanese Patent No. 3062753 discloses a technique developed by the inventors for producing high-quality single crystals in a safe and simple manner. Japanese Patent No. 3089418 discloses lithium calcium aluminum fluoride serving as a laser single crystal. Japanese Patent No. 3168294 discloses a barium lithium fluoride single crystal. In addition, lithium calcium aluminum fluoride has been reported to have characteristics. suitable for use as a scintillator, in “Scintillation decay of LiCaAlF6:Ce3+ single crystals,” M. Nikl, N. Solovieva, E. Mihokova, M. Dusek, A. Vedda, M. Martini, K. Shimamura, and T. Fukuda, Phys. Stat. Sol. (a) 187 (2001) R1–R3;and “LiCaAlF6:Ce crystal: a new scintillator,” A. Gektin, N. Shiran, S. Neicheva, V. Gavrilyuk, A. Bensalah, T. Fukuda, and K. Shimamura, Nuclear Instruments and Methods in Physics Research A 486 (2002) 274–277. However, lithium calcium aluminum fluoride has the problem that the density is as low as 2.94 g/cm3 and the γ-ray absorption coefficient is small. Meanwhile, a cerium fluoride single crystal has a luminescence wavelength in a UV region. Since cerium fluoride can be produced in the form of colorless, transparent single crystals, no absorption due to scattering occurs, providing excellent characteristics in luminescence lifetime. Japanese Patent Application Laid-Open (kokai) No. 2000-290097 discloses a cerium fluoride single crystal that has been investigated as a promising candidate scintillator. However, the emission output is problematically low; i.e., half the output of BGO.